Catalysis by reversed micelles in non-polar solvents: aquation and electron-transfer reactions of chromium(III) and cobalt(III) complexes in benzene

Abstract

Rate constants for aquation of the ion [Cr(C₂O₄)₃]^3– by octylammonium tetradecanoate-solubilized water in benzene and for aquations of [Cr(C₂O₄)₃]^3–, [CO(C₂O₄)₃]^3–, and cis-[Co(en)₂(N₃)₂]⁺(en = ethylenediamine) by dodecylammonium propionate-solubilized water in benzene are factors of up to 5 × 10⁶, 1 × 10⁶, ca. 1 500, and 11, respectively, greater than those for the same reactions in water. At constant surfactant concentration there is a linear dependence of the aquation rate on the concentration of solubilized water, while at constant water concentration increasing surfactant concentration results in an exponential rate decrease. These results are discussed in terms of favourable substrate orientation in polar cavities of alkylammonium carboxylate aggregates, formed from these surfactants in benzene, where hydrogen bonding facilitates proton transfer and enhanced water activity accelerates synchronous M–O bond-breaking and nucleophilic attack by water. An implication of these results is that, in the hydrophilic environment of the reversed micelle, aquations of Co^III and Cr^III complexes follow rate-determining bimolecular mechanisms subsequent to formation of ‘one-ended dissociated’ species. No micellar effects have been observed for the electron-transfer process in the aquation of the ion [Co(C₂O₄)₂(H₂O)₂]^–. Rate enhancement by surfactant-solubilized water in non-aqueous solutions and the utility of these systems in elucidating the mechanistic roles of water are discussed.